CDL Combination Vehicles Practice Test

Updated October 27, 2026

1. In a set of doubles or triples, which trailer is most likely to turn over during a sudden lane change?
   A.The first trailer behind the tractorB.The last trailerC.The middle trailerD.All are equally likely
2. When you turn, the rear wheels of a trailer follow a path that is:
   A.Wider than the tractor's front wheelsB.Exactly the same as the front wheelsC.Shorter than the tractor's front wheels (off-tracking)D.Outside the tractor's path on the right side only
3. Why should you not use the trailer hand valve (trolley valve) to park?
   A.It can wear out the trailer tiresB.It drains the air tanksC.It is illegal in every stateD.Air can leak out, releasing the brakes, so always use the parking brakes
4. After coupling a tractor and trailer, how do you check that the coupling is secure?
   A.Pull gently forward in low gear against the locked trailer brakes, and visually check that the fifth wheel jaws are closed around the kingpin with no space between the upper and lower fifth wheelB.Look at it from the cab mirrorC.Drive at highway speed and listen for noisesD.Check that the landing gear is down
5. What does the tractor protection valve do?
   A.Protects the tractor's tires from overheatingB.Keeps air in the tractor's brake system if the trailer breaks away or develops a bad leakC.Locks the fifth wheelD.Raises the landing gear
6. Before backing under a trailer, the trailer should be at what height?
   A.High enough that the trailer lifts the fifth wheel fullyB.As low as possible, resting on the groundC.So that the trailer is lifted slightly when the tractor backs under itD.Height doesn't matter
7. What usually causes a trailer jackknife?
   A.Driving too slowlyB.Low tire pressure on the tractorC.Using the engine brake on a dry roadD.Locked trailer wheels that skid, often from braking too hard
8. Compared with straight trucks, fully loaded combination vehicles are:
   A.Less likely to roll overB.Much more likely to roll overC.Equally likely to roll overD.Impossible to roll over
9. What color is the trailer air supply control knob in most tractors?
   A.Blue, roundB.Yellow, squareC.Red, eight-sidedD.Green, triangular
10. What are glad hands?
    A.Gloves worn while couplingB.Hand signals used by a helperC.Handles on the fifth wheelD.Coupling devices that connect the service and emergency air lines from the tractor to the trailer
11. Before uncoupling, what should you do after making sure the area is clear?
    A.Lower the landing gear until it firmly contacts the groundB.Release the trailer brakesC.Raise the landing gearD.Drive forward quickly
12. When coupling, why should you check that the trailer is in line with the tractor before backing under it?
    A.To keep the trailer's air tanks fullB.To avoid hitting the trailer's landing gear or nose and to couple straightC.To check the tire pressureD.To charge the trailer battery
13. How should you steer a combination vehicle to reduce the risk of rollover?
    A.Make quick steering movementsB.Brake hard in the middle of turnsC.Steer gently and smoothly, and slow down before curves and rampsD.Change lanes suddenly to avoid traffic
14. Why should the tractor's fifth wheel plate be greased, as required?
    A.To make the trailer heavierB.To seal the air linesC.To keep the landing gear downD.To prevent steering problems from friction between the tractor and trailer
15. When uncoupling, why should you ease pressure on the locking jaws before releasing them?
    A.To make the locking jaws easier to releaseB.To check the brakesC.To raise the trailerD.To charge the air tanks